Many different processes are known for the microencapsulation of hydrophobic oils, particularly with colour formers of colour reaction recording systems dissolved therein. The thus obtainable microcapsules, which as a rule contain an oily hydrophobic solution of the colour former (a homogeneous substance) are generally applied in conventional manner to the underside of a sheet of paper, the CB sheet. The latter can be brought into contact with a so-called CF sheet, in which the surface facing the CB sheet is coated with a reactant which is acid for the colour former. The acid reactant can e.g. be an acid-treated clay, such as montmorillonite clay, or also a low molecular weight phenolic resin, such as a phenol formaldehyde novolak resin. If the acid reactant is an acid compound soluble in oily hydrophobic liquids, then it can also be encapsulated. During the printing process, the said capsules are crushed by the printing types. Therefore the reactants come into contact with one another and supply a coloured character on the CF sheet. The two colour reactants can also be applied to the surface of a single sheet, in order to obtain a so-called autogenic system. It is then recommended to encapsulate both reactants, if the acid reactant is soluble in the oil, in order to prevent an undesired premature reaction of the reactants.
For the production of capsule envelopes of microcapsules provided for duplication purposes in colour reaction systems, numerous encapsulation processes are known. They are e.g. based on the use of gelatin coacervate, polyisocyanate, polyamide or aminoplast systems. The aforementioned process is based on an aminoplast system, in which a water-soluble, non-ionic melamine/formaldehyde precondensate is reacted with a water-soluble polymer reactive therewith, accompanied by the formation of the capsule envelope. Increasing significance has been attached to this process of late. It can be relatively easily controlled and takes place under comparatively mild reaction conditions, so that even very sensitive colour formers remain undamaged during the reaction.
German Patent 35 45 803, which corresponds to U.S. patent application Ser. No. 922,591, now U.S. Pat. No. 4,824,823, describes a process of the aforementioned type in detailed form. This process leads to a particularly stable oil-in-water dispersion, in that the aqueous solution of a cationic melamine/formaldehyde precondensate and that of an organic polymer are mixed, accompanied by vigorous stirring, in order to bring about optimum turbulence and the exclusion of disturbing laminar conditions. As a result the originally dissolved, cationized melamine/formaldehyde precondensate is separated in ultra-fine suspension in the presence of a water-soluble polymer. These precipitated ultra-fine suspended particles leads to a particularly favourable stabilization of the oil-in-water dispersion, in which the capsule envelope is formed by condensation reactions with the water-soluble non-ionic melamine/formaldehyde precondensate as the main reactant.
The process described in German Patent 35 45 803 uses a water-soluble polymer in the first process stage, which is believed to interact with the cationic melamine precondensate to induce precipitation of the precondensate in ultra-fine suspension. It is to be assumed that this interaction is based on the presence of suitable and in particular Zerewitinoff-active hydrogen atom-carrying functional groups in the polymer. Such functional groups, which react with the melamine/formaldehyde precondensate, have long been known and in particular include acid amide amino imino ester ether hydroxyl urethane thiol or mercaptan groups. Preferred examples of the water-soluble polymers are polyvinyl alcohol, gelatin and maleic anhydride copolymers, particularly ethylene/maleic anhydride copolymers or styrene/maleic anhydride copolymers. Particular preference is given to acrylamide/acrylic acid copolymers, starch, cellulose derivatives such as carboxymethyl cellulose (CMC) or hydroxyethyl cellulose (HEC), alginates, such as sodium alginate, polyurethanes and polyethylene oxide. The acrylamide/acrylic acid copolymer has proved especially advantageous as a water-soluble organic polymer in this process, but constitutes a significant cost factor in the overall process of German Patent 35 45 803.